1. Technical Field
The present teaching relates to method and system for analog circuits. More specifically, the present teaching relates to method and system for power supply load sharing and systems incorporating the same.
2. Discussion of Technical Background
Connecting the output of two or more power supplies together allows them to share a common load current. Load sharing has various advantages. Load sharing puts less thermal stress on each individual power supply's components, thus increasing the overall power system's reliability and lifetime. It allows smaller power supplies to be used in parallel to supply a larger load. In systems with time-varying load currents supplied by dynamically-managed parallel supplies, load sharing enables each supply to be operated at its peak power-conversion efficiency point. High-availability electronic systems usually employ an N+n configuration of power supplies, where N is the number of supplies that are required to supply the load current and n is the number of extra or redundant supplies. Load sharing is often an essential feature of such systems.
The division, or sharing, of the load current between the supplies depends on their individual output voltages and the connection resistance to the common load. This is called droop sharing. To prevent reverse current into a supply, which is called back-feeding, a diode can be added in series with each supply output. In this case, the diode voltage drop also influences the current sharing between supplies.
An active method of load sharing is implemented in a load sharing controller from Linear Technology Corporation. In this load sharing controller, the current from each power supply is monitored by sensing the voltage drop across a series current sense resistor. The load sharing controller compares this current sense signal against a share bus signal. The share bus signal indicates the load current needed per supply to maintain regulation of the load voltage. The load sharing controller then adjusts the output voltage of the supply via the supply's feedback network or trim input to match its current to the share bus, thus achieving load sharing.
Texas Instruments offers device UCC39002, designed to achieve load sharing by adjusting the power supply voltage. The share bus signal indicates the highest of all the supply currents. National Semiconductor Corporation provides a device that uses a share bus that is the average of all the supply currents.
Those traditional approaches to load sharing have some disadvantages. Although droop sharing is simple, sharing accuracy can be hard to control. While the back-feeding problem is solved with one or more diodes connected in series, the diode itself wastes power. Although the load sharing controller from Linear Technology Corp. and other existing controller devices solve these problems, designs based on such conventional technologies can be complicated because one has to accommodate the power supply loop dynamics into the load sharing loop dynamics, with each supply requiring custom loop stability compensation. Additionally, these controllers can only manipulate supplies with a trim/adjust pin or an accessible feedback network. This may not be readily available, or there may be noise injection concerns about routing this signal on a circuit board. Furthermore, the need to route the share bus signal to all the supplies also introduces a potential single point of failure.